The present invention relates to electrocoat coating compositions containing self-crosslinking polymers. The self-crosslinking polymers of the invention are epoxy resins having terminal primary amine groups and a multiplicity of carbamate groups located along the backbone.
Electrocoating, or electrodeposition coating, is widely used in the art for the application of polymeric coatings to electrically conductive substrates. In the electrocoat process, the substrate to be coated is used as one electrode in an electrical cell so that a current passed through the aqueous bath will deposit the coating onto the substrate. One of the advantages of electrocoat compositions and processes is that the applied coating composition forms a uniform and contiguous layer on all exposed surfaces regardless of shape or configuration of the article. This is especially advantageous when the coating is applied as an anticorrosive coating onto a substrate having an irregular surface, such as a motor vehicle body. The even, continuous coating layer over all areas of the metallic substrate provides maximum anticorrosion effectiveness.
Electrocoat compositions usually comprise an aqueous dispersion of a principal film-forming resin, such as an acrylic or epoxy resin, having ionic stabilization. The electrocoat composition is preferably a cathodic electrocoat, composition when corrosion resistance of the coated substrate is a concern. A cathodic electrocoat composition is produced by using a principal resin having amine functionality. The amine functionality is at least partially neutralized to produce the corresponding amine salt. A sufficient number of neutralizable amine groups are required to make a stable dispersion or emulsion.
For automotive or industrial applications where hard, durable electrocoat films are desired, the electrocoat compositions are formulated to be curable compositions. Usually, this is accomplished by including in the bath a crosslinking agent that can react with functional groups on the principal resin under the appropriate conditions and thus cure the coating. The crosslinking agent and the polymer that is used as the principle resin react to form an intractable, insoluble polymeric network. For example, the electrocoat compositions may comprise a blocked isocyanate crosslinker that unblocks and reacts with the principal resin upon application of sufficient heat. Isocyanate crosslinkers have been preferred by the industry because of the toughness and durability that they impart to the cured film. However, the isocyanate crosslinkers are usually prepared from toxic isocyanate monomers and thus present exposure hazards during manufacture. In particular, aromatic isocyanate monomers, such as toluene diisocyanate, are hazardous. It would therefore be desirable to have a curable electrocoat composition that does not have the toxicity problems associated with isocyanate crosslinkers, yet produces films with the same excellent physical properties. Additionally, the principal resin and the blocked isocyanate crosslinkers must be prepared in separate processes. The manufacture of the electrocoat compositions must include a blending step where the crosslinker and principal resin are blended before being dispersed. A self-crosslinking principal resin would also simplify the manufacturing process by eliminating the separate crosslinker synthesis and blending steps.
We have now discovered an improved method of producing electrocoat coating compositions and self-crosslinking polymers for electrocoat coating compositions. The self-crosslinking polymers of our invention are linear or slightly branched polymers having terminal primary amine groups. The incorporation of primary amine groups onto the ends of the polymer according to the invention results in improved bath properties and provides improved coating compositions. Furthermore, the instant process introduces the crosslinking groups in a controlled and reproducible way, allowing for greater control of the architecture of the resulting crosslinked film. The instant process also does not require a separate preparation of a crosslinking resin or a blending step to combine the crosslinker with the principal resin.